Method for removing corona discharge contaminants in electrophotography

ABSTRACT

In a method and apparatus for electrophotography in which use is made of photosensitive element including a photoconductive layer and an image forming surface which may be the surface of the photoconductive layer or the surface of a highly insulative layer integrally bonded to the surface of the photoconductive layer and wherein an electric charge is deposited on the image forming surface by means of corona discharge, a light image is projected upon the photosensitive element to form a latent image on the image forming surface, and the latent image is transfer printed onto a recording medium, means is provided to apply an organic solvent to the image forming surface to remove contaminants formed by the ionization of air by the corona discharge.

United States Patent 11 1 Kinoshita et al.

[ METHOD FOR REMOVING CORONA DISCHARGE CONTAMINANTS 1N ELECTROPHOTOGRAPHY [75] Inventors: Koichi Kinoshita, Narashino; Shiro Uehara, Tokyo; Hiroshi Nagame,

Tokyo; Kazukiyo Yamada, Tokyo, all of Japan [73] Assignee: Katsuragawa Denki Kabushiki Kaisha, Tokyo-to, Japan [22] Filed: Sept. 13, 1971 [21] Appl. No.: 179,805

[30] Foreign Application Priority Data 1 1 Nov. 6, 1973 3,128,683 4/1964 Rubin 355/15 3,576,624 4/1971 Matkam 355/15 UX 3,598,487 8/1970 Miguguchi 355/15 OTHER PUBLICATIONS Primary Examiner-Norman G. Torchin Assistant Examiner-John R. Miller Attorney-Bosworth, Sessions, Herrstrom & Cain [57] ABSTRACT In a method and apparatus for electrophotography in which use is made of photosensitive element including a photoconductive layer and an image forming surface which may be the surface of the photoconductive layer or the surface of a highly insulative layer integrally bonded to the surface of the photoconductive layer and wherein an electric charge is deposited on the image forming surface by means of corona discharge, a light image is projected upon the photosensitive element to form a latent image on the image forming surface, and the latent image is transfer printed onto a recording medium, means is provided to apply an organic solvent to the image forming surface to remove contaminants formed by the ionization of air by the corona discharge.

9Claims, 3 Drawing Figures METHOD FOR REMOVING CORONA DISCHARGE CONTAMINANTS IN ELECTROPHOTOGRAPHY BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for electrophotography and more particularly to a method of cleaning the image forming surface of a photosensitive element for use in electrophotography to remove contamination and prevent deterioration caused by corona discharge.

Photosensitive elements presently used in the art of electrophotography can be classified into two types, one comprising a photoconductive layer and the other comprising a highly insulative layer and a photoconductive layer integrally bonded to the highly insulative layer. Latent images are formed on the surface of the photoconductive layer or of the highly insulative layer depending upon the method of forming the latent image. While a number of methods have been proposed to deposit an electric charge of given polarity on the image forming surface of the photosensitive element, the method which uses corona discharge is most suitable because it is not necessary to use an electrode in contact with the image forming surface and because it is possible to apply electric fields of high potential. In Xerography a charge is applied only once, while in the method developed by the applicant, electric fields of opposite polarity are applied consecutively. Thus, according to the latter method, a latent image is formed by the steps of applying a first electric field across a photosensitive element including a transparent highly insulative layer, a photoconductive layer exhibiting persistent internal polarization, and an electrode layer which are laminated in the order mentioned and are bonded together into an integral structure whereby to deposit a charge of one polarity on the surface of the highly insulative layer; applying a second electric field across the photosensitive element to deposit a charge of the opposite polarity and projecting a light image upon the photoconductive layer concurrently with the application of the second field whereby to form an electrostatic latent image corresponding to the light image on the surface of the highly insulative layer. The latent image is then developed under ambient light and transfer printed onto a recording medium. Alternatively, the latent image is often transferred onto an insulative film without developing. After cleaning, the photosensitive element can be used repeatedly many times. One problem encountered in the repeated use of the photosensitive element is hysteresis which is caused by charge remaining in the photoconductive layer. This problem has been solved. However, we have recently found that the image forming ability of the photosensitive element gradually declines due to wear of the image forming surface and to the deposition of contaminants formed by corona discharge and that this tendency is more pronounced where the latent image is transfer printed without developing. For this reason, the useful life of the photosensitive element is decreased, and the quality of the reproduced image is impaired.

We have found that the contaminants formed by the corona discharge are nitric acid and another acidic substances. More particularly, through the action of the corona discharge, the air near the discharge electrode is ionized to form N This gas and CO gas are conveyed to the image forming surface of the photosensitive element and react with moisture to form such acids as H CO and HNO Unstable acids such as H CO undergo decomposition and disappear in the form of CO, gas which does not cause any problem. On the other hand, HNO, gradually accumulates to form a strongly acidic film and to fix the moisture in the atmosphere on the image forming surface of the photosensitive element. Once the highly insulative layer of the photosensitive element is coated with such an acidic contaminant, its electrical resistance decreases which lowers its ability to preserve the latent image. As a result, the visible image produced by developing the latent image is not clear and in an extreme case it becomes impossible to produce the picture image.

Besides these chemical phenomena caused by the contaminants, physical phenomena were also observed. For example, when the highly insulative layer on the surface of the photosensitive element contains irregularities such as pin holes and cracks, the contaminants are deposited upon these irregularities thus rendering them electroconductive. Accordingly, ions migrating from the corona discharge electrode toward the photosensitive element tend to concentrate at these irregularities or defects of low resistance with the result that the peripheries of these defects on the highly insulative layer are damaged by concentrated ion bombardment. Further, the contaminants find their way into the photosensitive layer so as to decrease the resistance thereof at portions close to the defects. This increases the intensity of the electric field across the highly insulative 7 layer thus damaging the same.

When a latent image is transfer printed onto a recording paper before or after development, the contaminants are also transferred. Where the surface of the photosensitive element is not perfectly flat but contains irregularities as described above, certain amounts of the contaminants remain in the defects and cause ion concentra-tion during subsequent use of the photosensitive element.

The highly insulative layer of the photosensitive element is generally made of a thin film of synthetic material, typically a polyester resin. A film of polyester resin has sufficiently high electrical and mechanical strength and it has been believed that these properties are extremely stable. However, it was found that, for the reasons described above, these properties degrade relatively rapidly, especially in an atmosphere of high humidity.

We have made an accelerated test in which a photosensitive element was mounted on a rotary drum with the highly insulative layer facing outwardly and a pair of corona discharge units of opposite polarities were mounted along the periphery of the rotary drum. With continuous rotation of the drum the surface of the highly insulative film was charged with one polarity by one corona discharge unit and the charge was then neutralized by the charge supplied by the other corona discharge unit. After repeating several hundred cycles of this charging and neutralization operation, the contaminants deposited on the surface of the highly insulative material were collected and analysed. The result showed that the acid deposited consisted essentially of nitric acid and that the acidity often fell below pH 6. Similar results were also noted when the polarities of the corona discharge units were reversed. In the case of a photosensitive element used in Xerography, not

having the highly insulative layer, the same problem was noted.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved method and apparatus for electrophotography in which the photosensitive element can be used over long periods without deterioration caused by corona discharge.

Briefly stated, according to this invention in a method and apparatus for electrophotography of the type wherein a latent image is formed on the image forming surface of a photosensitive element including a photoconductive layer by depositing an electric charge on the image forming surface by means of corona discharge and projecting a light image upon the photoconductive layer, and the latent image is transfer printed onto a recording medium, an organic solvent is applied onto the image forming surface after the latent image has been transfer printed to remove contaminants formed by the ionization of air by the corona discharge and deposited on the image forming surface.

According to another aspect of this invention, there is provided a method of electrophotography of the type wherein a latent image is formed by the steps of applying a first electric field by means of corona discharge across a photosensitive element including a highly insulative layer and a photoconductive layer exhibiting persistent internal polarization and integrally bonded to the highly insulative layer so as to deposit a charge of one polarity on the surface of the highly insulative layer, applying a second electric field by means of corona discharge across the photosensitive element to deposit a charge of the opposite polarity on the surface of the highly insulative layer, and projecting a light image onto the photoconductive layer concurrently with the application of the second electric field thus forming a latent image corresponding to the light image on the surface of the highly insulative layer, and transfer printing said latent image onto a recording medium, characterized in that an organic solvent is applied onto the image forming surface of the highly insulative layer after the latent image has been transfer printed to remove contaminants formed by the ionization of air by the corona discharge and deposited on the image forming surface.

According to still further aspect of this invention, in apparatus for electrophotography of the type comprising a photosensitive element including a highly insulative layer and a photoconductive layer exhibiting persistent internal polarization and integrally bonded to said highly insulative layer, a rotary drum for supporting the photosensitive element with the highly insulative layer facing outwardly, a first corona discharge unit adapted to deposit a charge of one polarity on the surface of the highly insulative layer, a second corona discharge unit adapted to deposit a charge of the opposite polarity on the surface of the highly insulative layer, an optical system for projecting a light image onto the photoconductive layer concurrently with the operation of the second corona discharge unit whereby to form a latent image corresponding to the light image on the surface of the highly insulative layer, and means for transfer printing the latent image onto a recording medium, there is provided means for applying an organic solvent onto the image forming surface of' the highly insulative layer after the latent image has been transfer printed to remove contaminants formed by the ionization of air by the corona discharge and deposited on the image forming surface.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENT To enable better understanding of the invention the following examples are given.

EXAMPLE 1 The photosensitive element shown in FIG. 1 comprises a highly insulative layer 1 having a thickness of 9 microns and made of a polyester resin sold under the trade name of Mylar, a photoconductive layer 2 exhibiting persistent internal polarization and an electrode layer 3 of thin aluminum foil. Any organic or inorganic photoconductive material manifesting persistent internal polarization can be used to form the photoconductive layer but in this example the layer 2 was prepared by uniformly dispersing a fine powder of CdS in a polyvinyl acetate binder. Layers 2 and 3 were bonded together into an integral structure and the photo-sensitive element was mounted on a rotary drum with the highly insulative layer facing outwardly. The photosensitive element is of the well known construction and is especially suitable for use in the method of electrophotography wherein first and second electric fields of opposite polarities are applied consecutively across the photosensitive element and an information containing light image is projected upon the photoconductive layer concurrently with the application of the secondfield so as to form a latent image on the surface of the highly insulative layer.

Thus, as shown in FIG. 2 the photosensitive element was mounted on a rotary drum 4 with the highly insulative layer 1 facing outwardly. When the rotary drum 4 is made of metal, the electrode layer 3 may be omitted as in FIG. 2. The drum was rotated in the clockwise direction as shown by the arrow, and around the periphery of the drum were disposed a first corona discharge unit 5 adapted to deposit a charge of one polarity on the surface of the highly insulative layer, a second corona discharge unit 6 adapted to deposit a charge of the opposite polarity, a source of uniform light 7, a transfer printing roller 8 for urging an insulative recording paper 10 against the surface of the photosensitive element to transfer print the latent image onto the recording paper 10 and a roller 9 for applying a solvent onto the image forming surface of the highly insulative layer 1 to remove contaminants therefrom. There was also provided an optical system shown as a lens 11 for projecting a light image of an object 12 illuminated by lamps l3 and moved in synchronism with the rotation of the photosensitive element through the second corona discharge unit 6 concurrently with the operation of the same.

As shown in FIG. 3, roller 9 for removing the contaminants comprises a roller 9b covered with a layer of felt 9a and a pair of rollers 90 and 9d urged against the layer of felt. These rollers are immersed in an organic solvent 9f such as trichlene (trichloroethylene) contained in a container 9e. Rollers 9c and 9d are used to control the quantity of the solvent carried by felt layer 9a. During operation, when drum 4 is rotated in the direction of the arrow, the roller 9 is also rotated in the direction shown by the arrow in FIG. 3 to continuously clean the image forming surface of the photosensitive element with the solvent. With this electrophotographic apparatus, latent images were repeatedly formed on the surface of the highly insulative layer 1 and transfer printed ontothe recording paper 10. Up to the 70,000th cycle, clear picture images were always formed. However, beyond 70,000 cycles local defects caused by pin holes began to appear, thus slightly degrading the quality of the reproduced copy. However, no adverse effect due to contaminants caused by corona discharge was noted.

To demonstrate the effectiveness of the invention, an experiment was made without using roller 9 and it was found that beyond 2,000 cycles the resolution of the transfer printed images gradually decreased. At about the 3,000th cycle the resolution of the image was completely lost. This shows that the useful life of the photosensitive element was improved remarkably by practicing the present invention.

EXAMPLE 2 A photosensitive element having the same construction as in example 1 was mounted on a rotary metal drum. The assembly was mounted in a vacuum tank evacuated to 10 mm Hg or higher. The metal drum was grounded and a high frequency voltage of 3KHz was applied across the metal drum and an electrode disposed in the vacuum tank. Then vapor of styrene monomer was admitted into the vacuum tank where the monomer was polymerized by the action of the high frequency electric field and the resulting polymer was deposited on the image forming surface of the photosensitive element. After treatment for IS minutes, the assembly of the photosensitive element and the rotary drum was taken out of the vacuum tank and was used to repeatedly form and transfer print latent images as shown in FIG.2. In this example, no tendency toward degrading the quality of the transfer printed latent images or visible images reproduced therefrom was noted up to 120,000 cycles or more. It is presumed that this improved result is caused by the fact that the defects, such as pin holes or scratches, on the image forming surface of the highly insulative layer are filled with the styrene polymer. In this manner, by coating the defects on the image fonning surface with a highly insulative material resistant to the solvent utilized for the purpose of removing the contaminants, the useful life of the photosensitive element can be greatly prolonged. Such insulative material can also be applied by spraying, brushing or dipping and the thickness thereof may be very small.

The solvent utilized for removing the contaminants may be any organic solvent that can dissolve and remove mainly HNO Among these organic solvents are trichlene (trichloroethylene), benzene, carbon tetrachloride and methyl alcohol. Especially, trichlene and carbon tetrachloride are preferred because of their high insulating strength. The solvents must not dissolve the highly insulative layer of the photosensitive element. Also it is desirable that the solvents be highly insulative and also highly evaporative so that the latent image can be formed on a dry image forming surface, the solvent is not always required to be highly evaporative since the latent image can be readily formed and transfer printed under wet conditions.

The roller 9 may be positioned at any desired location around the periphery of the photosensitive element, and may be brought into contact with the image forming surface continuously or intermittently. Further, it is to be understood that the roller may be replaced by, for example, a replaceable brush of cloth, sponge or felt impregnated with the solvent.

Although the invention has been described in terms of a method and apparatus for electrophotography utilizing a photosensitive element including a highly insulative layer and a photoconductive layer manifesting persistent internal polarization and wherein the latent image is formed on the image forming surface of the highly insulative layer by consecutive application of first and second electric fields and projection of a light image concurrently with the application of the second electric field, it should be understood that the invention can also be applied to ordinary Xerography wherein a photosensitive element including a photoconductive layer is used and the latent image is formed on the image forming surface of the photoconductive layer by first depositing a uniform electric charge by means of corona discharge and then projecting a light image onto the photoconductive layer. The effect of the contaminants on this type of photosensitive element is more serious because it is not provided with a highly insulative layer which is smoother and more resistant to acids than the bare surface of the photoconductive layer.

We claim:

1. In a method of electrophotography of the type wherein a latent image is formed on the image forming surface of a photosensitive element including a photoconductive layer by depositing an electric charge on said image forming surface by means of corona discharge and projecting a light image upon said photoconductive layer, and the latent image is transfer printed onto a recording medium, the improvement which comprises the step of applying an electrically insulative organic solvent onto said image forming surface after said latent image has been transfer printed to remove contaminants formed by the ionization of air by said corona discharge and deposited on said image forming surface.

2. In a method of electrophotography wherein a latent image is formed by the steps of applying a first electric field by means of corona discharge across a photosensitive element including a highly insulative layer and a photoconductive layer exhibiting persistent internal polarization and integrally bonded to said highly insulative layer so as to deposit a charge of one polarity on the surface of said highly insulative layer, applying a second electric field by means of corona discharge across said photosensitive element to deposit a charge of the opposite polarity on the surface of said highly insulative layer, and projecting a light image onto said photoconductive layer concurrently with the application of said second electric field thus forming a latent image corresponding to said light image on the surface of said highly insulative layer, and said latent image is transfer printed onto a recording medium, the improvement which comprises applying an electrically insulative organic solvent onto the image forming surface of said highly insulative layer after said latent image has been transfer printed to remove contaminants formed by the ionization of air by said corona discharge and deposited on said image forming surface.

3. The method according to claim 2 wherein said solvent is applied by means of a roller in contact with said image forming surface.

4. The method of electrophotography according to claim 1 wherein said contaminants comprise nitric acids.

5. The method of electrophotography according to claim 2 wherein said contaminants comprise nitric acids.

6. The method of electrophotography according to claim 1 in which a thin film of a highly insulative material resistant to said solvent is applied upon said image forming surface prior to fonnation of a latent image thereon.

7. The method of electrophotography according to claim 6 wherein said thin film is formed by polymerizing vapor of a styrene monomer by the action of a high frequency electric field and causing a resulting styrene polymer to deposit on said image forming surface.

8. The method of electrophotography according to claim 2 in which a thin film of a highly insulative material resistant to said solvent is applied upon said highly insulative layer prior to formation of a latent image thereon.

9. The method of electrophotography according to claim 8 wherein said thin film is formed by polymerizing vapor of a styrene monomer by the action of a high frequency electric field and causing a resulting styrene polymer to deposit on said highly insulative layer. 

2. In a method of electrophotography wherein a latent image is formed by the steps of applying a first electric field by means of corona discharge across a photosensitive element including a highly insulative layer and a photoconductive layer exhibiting persistent internal polarization and integrally bonded to said highly insulative layer so as to deposit a charge of one polarity on the surface of said highly insulative layer, applying a second electric field by means of corona discharge across said photosensitive element to deposit a charge of the opposite polarity on the surface of said highly insulative layer, and projecting a light image onto said photoconductive layer concurrently with the application of said second electric field thus forming a latent image corresponding to said light image on the surface of said highly insulative layer, and said latent image is transfer printed onto a recording medium, the improvement which comprises applying an electrically insulative organic solvent onto the image forming surface of said highly insulative layer after said latent image has been transfer printed to remove contaminants formed by the ionization of air by said corona discharge and deposited on said image forming surface.
 3. The method according to claim 2 wherein said solvent is applied by means of a roller in contact with said image forming surface.
 4. The method of electrophotography according to claim 1 wherein said contaminants comprise nitric acids.
 5. The method of electrophotography according to claim 2 wherein said contaminants comprise nitric acids.
 6. The method of electrophotography according to claim 1 in which a thin film of a highly insulative material resistant to said solvent is applied upon said image forming surface prior to formation of a latent image thereon.
 7. The method of electrophotography according to claim 6 wherein said thin film is formed by polymerizing vapor of a styrene monomer by the action of a high frequency electric field and causing a resulting styrene polymer to deposit on said image forming surface.
 8. The method of electrophotography according to claim 2 in which a thin film of a highly insulative material resistant to said solvent is applied upon said highly insulative layer prior to formation of a latent image thereon.
 9. The method of electrophotography according to claim 8 wherein said thin film is formed by polymerizing vapor of a styrene monomer by the action of a high frequency electric field and causing a resulting styrene polymer to deposit on said highly insulative layer. 